In the field of high fidelity music reproduction systems, substantial and continuous efforts are being made to obtain sound reproduction which is as faithful as possible to the original sounds to be reproduced by the high fidelity system. The problem is most acute for lower frequency sounds, otherwise known as bass sounds; and most speaker systems fail to faithfully reproduce such bass sounds. In many systems, the inherent resonant frequency of the speaker and the enclosure with which it is combined function to produce "booming" or unnatural emphasis of bass tones of particular frequencies, while de-emphasizing other frequencies.
In most systems, tones are reproduced by a diaphragm of the loudspeaker which is moved by a combination of a permanent magnet and voice coil to which the tone signals representative of the sound are applied. As the diaphragm vibrates, sound waves are propagated by it both in front of and behind the loudspeaker. The signals on opposite sides of the loudspeaker are 180.degree. out of phase with each other, so that provisions must be made for preventing the sound wave at the rear of the speaker from cancelling the front sound wave. This is accomplished by mounting the speaker in an appropriate enclosure which is employed to contain or dissipate the rear sound wave and permit free radiation of the front sound wave from the speaker.
The rear sound wave is useful only if it is in phase or augments the front sound wave, and various complex baffle speaker arrangements have been developed to accomplish such an inversion of the rear sound wave. In most cases, however, speaker systems which use this principle have a characteristic of peaking or booming at certain frequencies and consequently produce an unrealistic response over the range of frequencies reproduced by such systems.
Many prior art speaker enclosures are operated on the principle of an "infinite baffle" which ideally completely separates the rear sound wave from the front sound wave. In such a speaker, it is desirable to have the enclosure as large as possible; and since there are definite practical limitations to accomplishing this result, such speakers generally compromise the size of the speaker enclosure by internal baffles, filling the speaker enclosure with sound deadening materials, such as fiber glass and the like, and perforating the back of the speaker enclosure.
Another type of speaker enclosure which has achieved substantial popularity in recent years is a sealed air or acoustic suspension speaker enclosure. This type of enclosure utilizes an air tight cabinet or box in contrast to bass reflex or infinite baffle systems which are not air tight. Such acoustic suspension systems can employ speakers having a diaphragm which is more elastic than the diaphragms used in bass reflex speaker cabinets since the restoring force for the diaphragm is provided in large part by the confined air mass behind the speaker and does not depend so much on the mechanical elastic suspensions of the diaphragm itself, particularly at large diaphragm excursions. By utilizing a loudspeaker of reduced elastic stiffness, improved linearity of response has been attained when the speaker is properly matched to its cabinet enclosure. A primary disadvantage of acoustic suspension systems, however, is that they are relatively inefficient, requiring higher power amplifiers than bass reflex systems.
In spite of the relative sophistication of the speaker/enclosure art, the tones reproduced by speaker systems, either of the acoustic suspension type or the bass reflex type (infinite baffle) are not as realistic as desired by serious listeners. Much of the distortion which is produced by conventional speaker cabinet enclosures is believed to be produced by the flat parallel surfaces on the interiors of such enclosures. The radiating sound waves within the enclosures strike these surfaces and bounce back and forth between them to set up standing waves within the enclosure. These standing waves interfere with the proper operation of the speaker in response to various frequencies across its range of response. Non-linear response characteristics and intermodulation distortion are quite noticeable even in the best of the prior art speaker systems.
The problems which are presented by flat surfaces on the inside of a speaker enclosure have been recognized in the past, and attempts have been made to overcome them. To avoid establishing standing waves within a speaker enclosure, speaker enclosures have been manufactured having a spherical or elliptical inner surface behind the speaker. By utilizing such curved inner surfaces in the speaker interior, the development of standing waves within the enclosure has been minimized. In the past, however, the development of such spherical speaker enclosures has been directed to infinite baffle speaker enclosures which require relatively large internal air volumes to achieve the desired operating results. As a consequence, spherical speaker enclosures have not achieved any notable degree of commercial acceptance.
It is desirable to provide a speaker enclosure which produces realistic sound reproduction without unnatural frequency argumentation at particular frequencies which is small in size, relatively inexpensive to manufacture, and which has improved efficiency.